Message contains NUL characters - howto dump?

[Klemens Puritscher]

Hello,

I have a problem with one of our customers.
When he forwards an email with the thunderbird email client (windows version), the lmtp-daemon on my cyrus-imapd (v2.3.13) rejects those emails with the error "554 5.6.0 Message contains NUL characters".
...ok, that's clear, there are "NUL" characters in the email.

But I would show my customer, where the "NUL" character is.

For tests, I generate a testmail, with "echo -e "From:<sender at example.com>\nTo:<recipient at example.com>\nSubject: test\n\ntest\0000test\n.\n" > mail_with_NUL.txt

Now I dump the lmtp-session on the cyrus-imapd host with:
tcpdump -vv -XX -s 65535 -n -i eth1 "port lmtp

and I see the "NUL" character:
...
        0x0230:  7065 6564 2e61 740d 0a0d 0a74 6573 7400  peed.at....test.
        0x0240:  7465 7374 0d0a 2e0d 0a                   test.....
...
65 = e
73 = s
74 = t
00 = NUL

...ok, fine, I can find the "NUL" character.

But when I dump the lmtp-session with the customer email (which get's the error "554 5.6.0 Message contains NUL characters"), I cannot find this "NUL" character.

Can someone tell me, what I did wrong?

Thanks in advance.

Klemens